Hi from a purely personal experience, the benefit of Tinky over the stock embedded OS included in my NAS (DUAL SATA) was:
1. Install additional IPKG packagas
2. More useful packages included by default, such as SSH server.
3. Most importantly, from my perspective, Tinky allows one to persist changes to configuration files and other aspects on the filesystem. For example startup scripts, cron jobs and so on. This is achieved by saving data to disk before reboot, and then coping them back in the RAMDISK when it reboots. Very clever
I never installed Debian/Gentoo on my DUAL Sata NAS, but I suppose doing so would give you the benefits of having a full Linux disto, with access to their package management repositories.